Stokes parameter measurement device and method

ABSTRACT

An object is to accurately measure the Stokes parameters, without the occurrence of polarization fluctuations or PDL during the splitting of the incident light. When the incident light is made incident on a first-stage prism, the light is split into two first splitting light rays. Next, the first split light rays are respectively incident on a pair of prisms of a second stage. Each of the pair of first split light rays is split into two rays by a second-stage prism, to obtain four second split light rays.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a Stokes parameter measurement devicewhich measures the state of polarization of signal light or other light,and the measurement method of same.

[0003] 2. Description of the Related Art

[0004] Stokes parameters are parameters which represent a state ofpolarization. When measuring Stokes parameters, the incident signallight is split into four rays using a splitter means such as a beamsplitter, half-mirror, or filter; each of the signal light rays is givena different polarization and phase by means of a half-wave plate orother polarizer and a quarter-wave plate or other phase shifter, theoptical components of each of the split signal light rays are subjectedto photoelectric conversion using photodetector elements, and operationsare performed on the electrical components obtained by photoelectricconversion to obtain the Stokes parameters. The configuration of aStokes parameter measurement device such as that described above isdisclosed in Japanese Patent Application Laid-open No. 6-18332.

[0005] In such a conventional Stokes parameter measurement device, abeam splitter, half-mirror, filter, or other device is used as splittermeans to split the incident light. Using such splitter means, becausethe incident light is split by means of interference, polarizationfluctuations and a PDL (polarization-dependent loss) occur. As a result,there is the problem that high-precision measurement of Stokesparameters is difficult.

SUMMARY OF THE INVENTION

[0006] This invention was devised in light of the above problems withthe prior art, and has as an object the provision of a device and methodfor the accurate measurement of Stokes parameters, without theoccurrence of polarization fluctuations or PDL during splitting of theincident light.

[0007] A further object of this invention is to provide a Stokesparameter measurement device comprising an incidence portion, on whichthe signal light for measurement is incident; an optical splitterportion, having at least one prism, and which splits signal light whichhas passed through the incidence portion into at least four split rays;a phase compensation portion, which applies different polarizations andphases to each of the split signal light rays; and a photodetectorcircuit portion, which converts the optical component of the lightsignal output from the phase compensation portion into an electricalsignal. It is preferable that, in the above Stokes parameter measurementdevice, an operation portion is provided which performs operations onthe electrical component signal obtained by photoelectric conversion toobtain the optical intensity, 0° linear-polarization component, 45°linear-polarization component, and right-circular-polarizationcomponent, which are the Stokes parameters.

[0008] In the above Stokes parameter measurement device, signal lightwhich has passed through the incidence portion is split into at leastfour rays by the optical splitter portion having at least one prism,without using interference; hence polarization fluctuations and PDL caneasily be suppressed, and high-precision measurements of Stokesparameters can be performed.

[0009] In a preferred aspect of the above device, the above opticalsplitter portion causes the signal light which has passed through theabove incidence portion to be incident on two faces containing an edgeformed in the above one or more prisms, to split the signal light. Inthis case, the prism edge can be used to obtain split light rays in anarbitrary splitting ratio.

[0010] In another preferred aspect of the above device, the aboveoptical splitter portion comprises a single prism having aquadrangular-pyramid shaped light-receiving portion on the incidenceside; signal light having passed through the above incidence portion iscaused to be incident on the four side faces of this light-receivingportion, to split the signal light into four rays. In this case, thesignal light can be split into four rays by means of a simpleconfiguration using a single prism. This single prism has aquadrangular-pyramid shaped light-emission portion on the emission side;by arranging the four side faces of the above light-receiving portion tobe parallel respectively to the opposing four side faces of the abovelight-emission portion, the light split into four rays can be caused tobe emitted at once, all parallel to the incident light.

[0011] In still another preferred aspect of the above device, the aboveoptical splitter portion comprises one or more prisms, with opposingfaces formed in parallel. In this case, split light rays emittedparallel to the incident light can be obtained.

[0012] In still another preferred aspect of the above device, the abovephase compensation portion comprises a phase shifter and a polarizer.

[0013] In still another preferred aspect of the above device, acondensing lens is positioned between the above phase compensationportion and the above photodetector circuit portion.

[0014] In still another preferred aspect of the above device, the aboveoptical splitter portion comprises a first splitter portion and a secondsplitter portion; the first splitter portion comprises a single prism,and the second splitter portion comprises two prisms.

[0015] In still another preferred aspect of the above device, the aboveoptical splitter portion further comprises a dividing portion whichcauses signal light which has passed through the above incidence portionto be partially emitted, without passing through the above phasecompensation portion or the above photodetector circuit portion. In thiscase, signal light being measured by the Stokes parameter measurementdevice can be monitored using another device.

[0016] In still another preferred aspect of the above device, the aboveoptical splitter portion comprises a wavelength dispersion correctionportion, which cancels the wavelength dependence of the emissionposition of signal light split by the above one or more prisms. In thiscase, Stokes parameters can be measured over a plurality of wavelengths,without modifying the placement of the photodetector circuit portion orother portions.

[0017] A further object of this invention is to provide a Stokesparameter measurement method in which the signal light for measurementis caused to be incident from an incidence portion; the incident signallight is split into four rays by an optical splitter portion, comprisedof one or more prisms; each of the split signal light rays is endowedwith a different polarization and phase by the phase compensationportion; the optical component of the signal light from a phasecompensation portion is photoelectrically converted into an electricalsignal by a photodetector circuit portion; and operations are performedby an operation portion on the electrical component signal convertedfrom the optical signal to obtain the optical intensity, 0°linear-polarization component, 45° linear-polarization component, andright-circular-polarization component, which are the Stokes parameters.

[0018] In the above Stokes parameter measurement method, polarizationfluctuations and PDL can easily be suppressed, so that high-precisionmeasurement of Stokes parameters is possible.

[0019] In a preferred aspect of the above method, the above opticalsplitter portion comprises a first splitter portion and a secondsplitter portion; signal light incident from an incidence portion issplit into two rays in the first splitter portion, and the signal lightsplit into two rays is further split into two rays each in the secondsplitter portion.

[0020] In a preferred aspect of the above method, in the above opticalsplitter portion, signal light which has passed through the aboveincidence portion is caused to be incident on two faces containing anedge formed in the above one or more prisms, to split the signal light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a block diagram showing the configuration of themeasurement device of a first embodiment;

[0022]FIG. 2 shows one example of the configuration of the opticalsplitter portion, phase compensation portion, and photodetector circuitportion of FIG. 1;

[0023]FIG. 3 shows split optical paths within a prism;

[0024]FIGS. 4A to 4D show variations of the prism structure used in thesplitter optical path of FIG. 2;

[0025]FIG. 5 explains the method for performing compensating calibrationin the measurement device of FIG. 1;

[0026]FIG. 6 is a portion of the optical splitter portion in themeasurement device according to a second embodiment of the presentinvention, and shows the placement of lenses in the optical splitterportion;

[0027]FIG. 7 shows the state of arrangement of lenses between the phasecompensation portion and the photodetector circuit portion;

[0028] in FIG. 8A is a perspective view and FIG. 8B is a side viewexplaining the construction of the optical splitter portion of themeasurement device according to a third embodiment of the presentinvention;

[0029] in FIGS. 9A and 9B are perspective views explaining theconstruction of an optical splitter portion of the measurement deviceaccording to a fourth embodiment of the present invention;

[0030] in FIG. 10A and 10B are perspective views explaining theconstruction of an optical splitter portion of the measurement deviceaccording to a fifth embodiment of the present invention; and,

[0031]FIG. 11 is a side view explaining the construction of an opticalsplitter portion of the measurement device of a sixth embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Below, aspects of this invention are explained, referring to thedrawings.

[0033] (First Embodiment)

[0034]FIG. 1 is a block diagram showing the configuration of the Stokesparameter measurement device of a first embodiment.

[0035] The Stokes parameter measurement device α of this examplecomprises an input portion 1, which is an incidence portion comprising areceptacle, collimating lens or similar; a polarization analyzer opticalportion A0; an electrical circuit portion B0; an output portion 2comprising a GP-IB or other. The polarization analyzer optical portionA0 comprises an optical splitter portion A1 and a phase compensationportion A2; the electrical circuit portion B0 comprises a photodetectorcircuit portion B1, employing a photodiode or similar, an operationcircuit portion B2, and an A/D conversion circuit portion B3. Though notshown, if necessary the A/D conversion circuit portion B3 may bereplaced with the analog output circuit portion.

[0036]FIG. 2 shows one example of the configuration of the opticalsplitter portion A1, phase compensation portion A2, and photodetectorcircuit portion B1 of FIG. 1.

[0037] The optical splitter portion A1 comprises three cuboidal prismsP1, P2, P3, arranged in a two-dimensional array. In this example, thefirst splitter portion comprises the prism P1, and the second splitterportion comprises the prisms P2 and P3. The former of these, prism P1,is positioned such that the vicinity of the center of the beam diameterof the incident light L1 passes through the edge of the apex angle.Consequently the incident light L1 is split with a splitting ratio(intensity ratio) of 1:1 by the two faces containing the edge of theprism P1. The pair of prisms P2 and P3 are positioned such that thevicinities of the centers of the beam diameters of the first split lightrays L2a, L2b emitted from the prism P1 pass through the edges of therespective apex angles.

[0038] Consequently, the first split light rays L2a, L2b emitted fromthe prism P1 are each split with a splitting ratio of 1:1 by the twofaces containing the edges of each of the prisms P2, P3.

[0039] In other words, the signal light splitting ratio is determined bythe positioning of each of the prisms P1, P2, P3, and by the portions ofthe prisms P1, P2, P3 on which the beam diameter centers of the incidentlight L1 and the first split light rays L2a, L2b are incident. Asdescribed above, in this example the incident light L1 and the firstsplit light rays L2a, L2b are made incident such that the beam diametercenters are positioned on the edges of the apex angles of the prisms P1,P2, P3, so that the second split light rays L3a, L3b, L3c, L3d are theresult of splitting the incident light equally into four parts, with asplitting ratio of 1:1:1:1.

[0040]FIG. 3 explains in detail the splitting of signal light in a prismP1. This prism P1 has a cuboid external shape, with a squarecross-section in the plane parallel to the plane of the paper. Thesignal light incident on the apex angle portion 41 of the prism P1, thatis, the incident light L1, is incident equally on the pair of side faces44, 45 containing the edge 42 formed in the apex angle portion 41, andis split into two in the plane parallel to the plane of the paper. Thesetwo split light rays each propagate along different optical paths withinthe prism P1, and are emitted separately as light emitted from the pairof opposing side faces 47, 48, that is, as the split light rays L2a,L2b. The pair of light rays which have been split and are emitted fromthe opposing side faces 47, 48 are refracted upon emission from theprism P1, becoming parallel to the incident light. The above explanationapplies to the prism P1; but in the prisms P2 and P3 also, the incidentlight rays, that is, the first split light rays L2a, L2b are split in amanner similar to that described above, and are emitted as emitted lightrays, that is, as the second split light rays L3a to L3d.

[0041] It is not necessary to set the splitting ratio for signal lightin each of the prisms P1 to P3 to 1:1. That is, the beam diametercenters of the incident light L1 and of the first split light rays L2a,L2b need not be positioned so as to be incident on the edge of the apexangles of the respective prisms P1, P2, P3. When the splitting ratio isnot 1:1, the splitting ratio is measured in advance, and compensatingcalibration data corresponding to the splitting ratio is provided to theoperation circuit portion B2.

[0042] In FIGS. 4A to 4D are drawings explaining variations of theprisms P1 to P3. That is, the prisms P1 to P3 shown in FIG. 2 arereplaced by prisms P with the shapes shown in A through D in FIGS. 4A to4D.

[0043] For example, prisms with a rhomboid-shape cross-section or with aparallelogram cross-section, as in FIGS. 4A and 4B, may be used. In sucha prism P with a rhomboid or parallelogram cross-sectional shape withparallel opposing faces, the emitted light rays L2a, L2b are alwaysparallel to the incident light ray L1, so that it is easy to positionpolarizers or phase shifter in the phase compensation portion A2.

[0044] As shown in FIG. 4C, the shape of the prism P may also have across-section which is substantially square, but with the cornersthrough which light does not pass removed. Using the shape of the prismin FIG. 4C, the device can be made more compact. And, as shown in FIG.4D, the prism P may have a shape in which the apex angle of the incidentportion is removed, so that when signal light is emitted from the prismP it is caused to branch into different directions.

[0045] Returning to FIG. 2, the phase compensation portion A2 comprisesa phase shifter C, comprising a quarter-wave plate or similar withpricipal axis direction at 0°; a polarizer D1, having a polarizationdirection angle of 0°; and a polarizer D2, having a polarizationdirection angle of 45°.

[0046] The principal axis direction of the phase shifter C andpolarization direction angles of the polarizers D1, D2 can be changed asappropriate, and are not particularly limited to the above principalaxis direction or polarization direction angles.

[0047] The phase shifter C is positioned so as to transmit only thelowermost second split light ray L3d among the second split light raysL3a, L3b, L3c, L3d split by the optical splitting portion A1; however,the phase shifter C can be positioned so as to transmit only one of theother second split light rays L3a to L3c instead. However, this phaseshifter C should be positioned in combination with the polarizer D2,described below, so that the placement of the polarizers D1, D2 must bechanged according to any changes in the placement of the phase shifterC.

[0048] The polarizer D1 is placed such that, among the second splitlight rays L3a, L3b, L3c, L3d resulting from splitting into four parts,only the second split light ray L3b passes through. However, the secondsplit light ray L3b passing through the polarizer D1 is assumed not topass through the phase shifter C. The polarizer D2 is placed such that,among the second split light rays L3a, L3b, L3c, L3d resulting fromsplitting into four parts, only the second split light rays L3c and L3dpass through. Of the two second split light rays L3c, L3d passingthrough the polarizer D2, one of the second split light rays, L3c, isincident on the polarizer D2 without passing through the phase shifterC, while the other second split light ray L3d passes through the phaseshifter C before being incident on the polarizer D2. Of the second splitlight rays L3a, L3b, L3c, L3d resulting from splitting into four parts,one of the second split light rays L3a does not pass through anything inthe phase compensation portion A2, but is incident on the photodetectorcircuit portion B1 without change.

[0049] The signal light rays L4a, L4b, L4c, L4d emitted from the phasecompensation portion A2 are incident on the respective photodetectorelements E1, E2, E3, E4 constructing the photodetector circuit portionB1, and the transmitted optical intensities of each of the signal lightrays are measured. As the photodetector elements E1, E2, E3, E4, forexample, photodiodes or other photoelectric conversion devices are used.

[0050] Next, the method of measurement of the Stokes parameter device ofthis embodiment is explained in detail, referring to FIGS. 1 and 2.

[0051] First, the incident light ray L1 from the incidence portion 1 isincident on the polarization analyzer portion A0. The incident light rayL1 is incident on the prism P1 of the optical splitter portion A1. Whenincident on the prism P1, the incident light ray L1 is split into twofirst split light rays L2a, L2b. Next, the first split rays L2a, L2b areincident on the prisms P2, P3, respectively. The first split rays L2a,L2b are then split into four second split rays L3a, L3b, L3c, L3d by theprisms P2, P3 respectively. In this embodiment, a method is used inwhich the incident light ray L1 which is the signal light is split intofour by the three prisms P1, P2, P3; hence the PDL and polarizationfluctuations which occur when performing splitting by conventional meansusing interference can be suppressed.

[0052] The four second split rays L3a, L3b, L3c, L3d are next incidenton the phase compensation portion A2. The first of the second split raysL3a does not pass through anything, and is incident on the photodetectorelement E1 in the same unchanged state, and the transmitted opticalintensity is measured. The second of the second split rays L3b passesthrough the polarizer D1, which has a polarization direction angle of0°, and is then incident on the photodetector element E2, so that thetransmitted optical intensity of the 0° linear polarization component ismeasured. The third of the second split rays L3c passes through thepolarizer D2 having a 45° polarization direction angle, and is incidenton the photodetector element E3, so that the transmitted opticalintensity of the 45° linear polarization component is measured. Thefourth of the second split rays L3d is first incident on a λ/4wavelength plate having the fast axis at 0°, and is then incident on thepolarizer D2 having a polarization direction angle of 45°, beforefinally being incident on the photodetector element E4, so that thetransmitted optical intensity of the right circularly-polarizedcomponent is measured.

[0053] If the four transmitted optical intensities are It, Ix, I45, andIq45, then it is known that the Stokes parameters S0, S1, S2, S3 can berepresented as in eq. (1) below. $\begin{matrix}\left. \quad \begin{matrix}{{S_{0} = {I\quad t}}\quad} \\{{S_{0} = {{2\quad I\quad x} - {I\quad t}}}\quad} \\{\quad {S_{0} = {{2\quad I\quad 4\quad 5} - {I\quad t}}}\quad} \\{\quad {S_{0} = {{2\quad I\quad q\quad 45} - {I\quad t}}}}\end{matrix} \right\} & (1)\end{matrix}$

[0054] That is, by measuring the four transmitted optical intensities,the Stokes parameters can be calculated.

[0055] For the transmitted optical intensities It, Ix, I45, Iq45, thevalues measured by photoelectric conversion are taken to be I₀,I₁,I₂,I₃in computing the Stokes parameters. The prisms are placed such that thesignal light splitting ratio results in four second split rays ofsubstantially equal optical intensity; however, depending on loss of thepolarizers, quarter-wave plate, quarter-wave plate wavelengthcharacteristics and other factors, intensity ratios may differ. Hence itis desirable that correction calculation functions be incorporated intothe operation circuit portion B2.

[0056] The splitting ratio may be measured in advance prior to insertingpolarizers and other components; but it is preferable that acompensating calibration method such as that of FIG. 5 be performed.That is, an input light source β capable of emitting completelypolarized light is prepared, and while changing the state ofpolarization of the completely polarized light from the input lightsource β via a polarization controller F, the light is input to theStokes parameter measurement device α. Because the light input to theStokes parameter measurement device α is completely polarized, for eachtransmitted optical intensity, it can be stipulated that the Stokesparameter at the maximum value is 1, and the Stokes parameter at theminimum value is −1. If the maximum values of I₀, I₁,I₂, I₃ arerespectively I_(0max), I_(1max), I_(2max), I_(3max), and the minimumvalues are I_(0min), I_(1min), I_(2min), I_(3min), then when the overallintensity is the transmitted intensity of the input light I₀, the Stokesparameter are as given by eq. (2). $\begin{matrix}\begin{matrix}{S_{0} = 1} \\{S_{i} = {{\frac{I_{i}}{I_{0}} \times \frac{I_{0\quad \max} + I_{0\quad \min}}{I_{i\quad \max} - I_{i\quad \min}}} - \frac{I_{i\quad \max} + I_{i\quad \min}}{I_{i\quad \max} - I_{i\quad \min}}}} \\\left( {{i = 1},2,3} \right)\end{matrix} & (2)\end{matrix}$

[0057] The degree of polarization (DOP) is calculated as in eq. (3):$\begin{matrix}{{DOP} = \frac{\sqrt{S_{1}^{2} + S_{2}^{2} + S_{3}^{2}}}{S_{0}}} & (3)\end{matrix}$

[0058] Hence the Stokes parameter measurement device can also be made tofunction as a DOP monitor.

[0059] In this way, the Stokes parameters and DOP value can becalculated; by means of this method, the PDL in the optical analysisportion can be kept small, so that a Stokes parameter measurementdevice, that is, a polarization analyzer, with good precision can beconfigured.

[0060] (Second Embodiment)

[0061] Below, the Stokes parameter measurement device of a secondembodiment is explained. In the measurement device of the secondembodiment, lenses to adjust the beam size are added to the polarizationanalyzer optical portion A0shown in FIG. 2.

[0062]FIG. 6 shows a portion of the optical splitter portion A1 in themeasurement device of the second embodiment. As is clear from thefigure, in the optical splitter portion A1, a beam expander lens 51 toenlarge the beam diameter of the incident light ray L1 and a collimatorlens 52 to cause the incident light ray L1 with enlarged beam diameterto be collimated before incidence on the prism P1 are positionedsequentially as a stage preceding the prism P1.

[0063]FIG. 7 shows an example in which condensing lenses 55, 56, 57, 58are placed between the photodetector circuit portion B1 and the phasecompensation portion A2. Here, after the second split light rays L3a,L3b, L3c, L3d have passed through the phase compensation portion A2,prior to incidence on the photodetector elements E1, E2, E3, E4, thesecond split light rays L3a, L3b, L3c, L3d are converged by thecondensing lenses 55, 56, 57, 58. By thus positioning these condensinglenses 55, 56, 57, 58, the light reception efficiency of thephotodetector elements E1, E2, E3, E4 is improved, and precision isstabilized.

[0064] (Third Embodiment)

[0065] Below, the Stokes parameter measurement device of a thirdembodiment is explained. In the measurement device of the thirdembodiment, the optical splitter portion A1 of FIG. 2 comprises a singleprism.

[0066] In FIG. 8A is a perspective view of the prism PQ comprised by theoptical splitter portion A1, and FIG. 8B is a side view of the prism PQ.In the optical splitter portion A1, the light-receiving portion of theprism PQ is a quandrangular-pyramid shape. The incident light L1 fromthe input portion 1 shown in FIG. 1 is incident on the apex portion 61of this pyramid shape, that is, on the four side faces 62 to 65containing four edges; as a result, a single prism PQ can split theincident light L1 into four split light rays L3a to L3d in a singleoperation. Hence the configuration of the optical splitter portion A1can be simplified.

[0067] In FIG. 9A shows a variation of the prism PQ of FIG. 8A and 8B.In this case, the incidence side and the emission side of the prism PQboth have a pyramid shape. In this prism PQ, by causing the incidentlight L1 to be incident on the four side faces 72 to 75 of the apexportion 71, four split light rays L3a to L3d can be caused to be emittedfrom the side faces 76 to 79 parallel to and opposing the side faces 72to 75. At this time, each of the split light rays L3a to L3d is parallelto the incident light ray L1, so that the design of the Stokes parametermeasurement device can be made compact. Also, by using such a prism PQ,the split light rays can be output with the same splitting ratio evenwhen the wavelength of the incident light is changed.

[0068]FIG. 9B shows another variation on the prism PQ shown in FIG. 9A.Here, a quandrangular column portion is formed between the pyramid shapeon the incidence side and the pyramid shape on the emission side. Inthis case, split light rays can be obtained with good efficiencyaccording to the refractive index of the prism PQ, and in addition thesize of the prism PQ in the direction perpendicular to the optical path,that is, the axis, can be reduced.

[0069] (Fourth Embodiment)

[0070] Below, the Stokes parameter measurement device of a fourth aspectis explained. The measurement device of the fourth aspect is a furthervariation of the measurement device of the third embodiment.

[0071] In FIG. 10A, the prism PP is a modification of the prism PQ shownin FIG. 9A, with the apex areas of the pyramid shapes on the incidenceside and emission side cut off, to form flat faces 80, 81 which aredividing portions. When the incident light L1 from the input portion 1shown in FIG. 1 is made incident on the incidence-side truncated pyramidof the prism PP in FIG. 10A, the incident light which is incident on thefour side faces 82 to 85 is split by these side faces 82 to 85 into fourrays, and emitted as the four split rays L3a to L3d from theemission-side truncated pyramid, that is, from the side faces 86 to 89parallel to and opposing the side faces 82 to 85. Incident light whichis incident on the flat face 80 in the center propagates rectilinearlywithout change, and is emitted, parallel to the split rays, from theopposing flat face 81 as the divided light LD. This divided light LD isutilized when using other measurement equipment to monitor the signallight during measurements in the Stokes parameter measurement device.That is, the prism PP of this embodiment, while having substantially thesame size as the prism PQ, also has the function of a coupler.

[0072] In FIG. 10B, a further variation on the prism PP of FIG. 10A isshown. In this case, a quandrangular column portion is provided betweenthe incidence-side truncated pyramid and the emission-side truncatedpyramid. Split light can be obtained with good efficiency according tothe refractive index of the prism PP, and in addition the size of theprism PP in the direction perpendicular to the optical path, that is,the axis, can be reduced.

[0073] (Fifth Embodiment)

[0074] Below, the Stokes parameter measurement device of a fifthembodiment is explained. The measurement device of the fifth aspect isanother variation on the measurement device of the first embodiment.

[0075] As shown in FIG. 11, an achromatic prism 90, which is awavelength dispersion correction portion, is added as a later stage ofthe prism P1 shown in FIG. 2. This achromatic prism 90 is aquandrangular column with a rhomboid cross-section. Also, the pair ofapexes of the achromatic prism 90 are positioned on the axis AX passingthrough the pair of apexes of the prism P1. Also, the refractive indexand dimensions of the achromatic prism 90 are adjusted such that splitlight is emitted from the same position of the achromatic prism 90,regardless of the wavelength of the incident light.

[0076] Incident light L1 which is incident on a side face 74 of theprism P1 is emitted as one of the split light rays from the side face 78parallel to and opposing the side face 74. At this time, due todispersion in the refractive index of the prism P1, split light with thelong wavelength λ1 propagates parallel to the axis AX close to the axisAX, and split light with the short wavelength λ2 propagates parallel tothe axis AX far from the axis AX.

[0077] Split light emitted from the opposing side face 78 propagatesrectilinearly and is incident on the side face 95 of the achromaticprism 90. The split light incident on the side face 95 propagates withinthe achromatic prism 90, and is output from the side face 99 parallel toand opposing the side face 95 as the split light ray L2a (L2b)propagating parallel to the axis AX. At this time, the refractive indexand dimensions of the achromatic prism 90 are adjusted such that thewavelength dependences of the emission position of split light in theprism P1 cancel; hence even when one of a pair of split light rayshaving different wavelengths λ1, λ2 is incident on the side face 95,split light is emitted from the same place of the opposing side face 99.Though not shown in the figure, split light which is incident on theother side face 75 of the prism P1 and split, and is emitted from theopposing side face 79, on passing from the side face 94 and opposingside face 98 of the achromatic prism 90 to be emitted, also has thewavelength dependence of the emission position canceled. Hence by usingsuch an achromatic prism 90, Stokes parameters can be measured across aplurality of wavelengths without modifying the positions of the phasecompensation portion A2 or photodetector circuit portion B1.

[0078] The shape of the prism P1 is not limited to that shown in FIG.11, but need only emit parallel split light. For example, the prism P1of FIG. 11 may be replaced with prisms of the types shown in FIG. 4A toFIG. 4C.

[0079] In the above explanation, the case in which an achromatic prism90 is positioned in the latter stage of the prism P1 was explained;however, the achromatic prism can also be provided in the latter stageof the two splitting prisms P2, P3 shown in FIG. 2. In this case, theachromatic prism is a quandrangular column prism having the samecross-sectional shape as shown in FIG. 11.

What is claimed is:
 1. A Stokes parameter measurement device,comprising: an incidence portion, on which the signal light to bemeasured is incident; an optical splitter portion, having at least oneprism, which splits signal light which has passed through said incidenceportion into at least four rays; a phase compensation portion, whichendows each of the split signal light rays with different polarizationsand phases; and, a photodetector circuit portion, which performsphotoelectric conversion of the optical component of the signal lightrays emitted from the phase compensation portion.
 2. The Stokesparameter measurement device according to claim 1, comprising anoperation circuit portion which performs operations on the electricalcomponent resulting from photoelectric conversion, to obtain an opticalintensity, 0° linear polarization component, 45° linear polarizationcomponent, and right circular polarization component, which are theStokes parameters.
 3. The Stokes parameter measurement device accordingto claim 1, wherein in said optical splitter portion, the signal light,having passed through said incidence portion, is caused to be incidenton two faces containing an edge formed in said at least prism, to splitthe signal light.
 4. The Stokes parameter measurement device accordingto claim 3, wherein said optical splitting portion comprises a singleprism having a quandrangular-pyramid shaped light-receiving portion onthe incidence side, and in which signal light, having passed throughsaid incidence portion, is caused to be incident on the four side facesof the light-receiving portion, to split the signal light into fourrays.
 5. The Stokes parameter measurement device according to any one ofclaims 1 to 4, wherein said optical splitter portion comprises at leastone prism formed such that opposing faces are parallel.
 6. The Stokesparameter measurement device according to any one of claims 1 to 4,wherein said phase compensation portion comprises a phase shifter andpolarizer.
 7. The Stokes parameter measurement device according to anyone of claims 1 to 4, further comprising a condensing optical lens,positioned between said phase compensation portion and saidphotodetector circuit portion.
 8. The Stokes parameter measurementdevice according to any one of claims 1 to 4, wherein said opticalsplitter comprises a first splitting portion and a second splittingportion; the first splitting portion comprises one prism; and the secondsplitting portion comprises two prisms.
 9. The Stokes parametermeasurement device according to any one of claims 1 to 4, wherein saidoptical splitter portion further comprises a division portion whichpartially emits signal light having passed through said incidenceportion, without passing through said phase compensation portion or saidphotodetector circuit portion.
 10. The Stokes parameter measurementdevice according to any one of claims 1 to 4, wherein said opticalsplitting portion further comprises a wavelength dispersion correctionportion, which cancels the wavelength dependence of the emissionposition of signal light split by said at least one prism.
 11. TheStokes parameter measurement device according to any one of claim 10,wherein said wavelength dispersion correction portion is formed from aprism.
 12. A Stokes parameter measurement method, in which signal lightfor measurement is made incident on an incidence portion; the incidentsignal light is split into four rays by an optical splitting portioncomprising at least one prism; each split signal light ray is endowedwith a different polarization and phase by a phase compensation portion;the optical components of the signal light rays emitted from the phasecompensation portion are subjected to photoelectric conversion by aphotodetector circuit portion; and operations are performed by anoperation circuit portion on the electric signals obtained byphotoelectric conversion, to calculate the optical intensity, 0°polarization component, 45° polarization component, and right circularlypolarized component, which are the Stokes parameters.
 13. The Stokesparameter measurement method according to claim 12, wherein said opticalsplitting portion comprises a first splitting portion and secondsplitting portion; signal light incident from the incidence portion issplit into two rays by the first splitting portion; and each of the twosplit rays of signal light are split into two rays by the secondsplitting portion.
 14. The Stokes parameter measurement method accordingto any one of claim 12 and 13, wherein, in said optical splitterportion, signal light having passed through said incidence portion iscaused to be incident on two faces containing an edge formed in said aleast one prism, to split the signal light.